This invention relates generally to coating compositions for medical devices. More particularly, the present invention relates to cross-linked compositions formed from a water insoluble copolymer having a bioresorbable region, a hydrophilic region and at least two cross-linkable functional groups per polymer chain. These compositions when placed in contact with an aqueous environment form hydrogels which are useful as sealants for porous materials and particularly for implantable prostheses. Furthermore, these hydrogels can be used as delivery vehicles for therapeutic agents. Medical devices coated and/or sealed with such hydrogels, processes for forming such devices and methods of making the hydrogels are also disclosed.
It is generally known to provide a porous substrate, such as an implantable prosthesis, with a biocompatible, biodegradable sealant or coating composition which initially renders the porous substrate fluid-tight. Over time, such a sealant composition is resorbed and the healing process naturally takes over the sealing function of the sealant composition as tissue ingrowth encapsulates the prosthesis. The art is replete with examples of naturally derived, as well as chemically synthesized sealant compositions.
Natural materials, such as collagen and gelatin, have been widely used on textile grafts. U.S. Pat. Nos. 4,842,575 and 5,034,265 to Hoffman Jr., et al. disclose the use of collagen as a sealant composition for grafts. More recently, co-owned and co-pending U.S. Ser. No. 08/713,801 discloses the use of a hydrogel or sol-gel mixture of polysaccharides for rendering fluid-tight porous implantable devices. Such sealant compositions are beneficial in that they are able to seal an implantable device without the need for chemical modification of the surface thereof and provide improved bioresorbability as the healing process occurs. Furthermore, fibrin, an insoluble protein formed during the blood clotting process, has also been used as a sealant for porous implantable devices.
The use of such biologically derived sealant compositions, however, suffers from several drawbacks. One such drawback is the difficulty in producing consistent coatings due to variations inherent in natural materials. Another drawback is that the body may identify such compositions as foreign and mount an immune response thereto. Thus, biologically-based sealant compositions can cause inflammation, as well as infection at or around the site of implantation, which can lead to life-threatening complications.
Accordingly, attempts have been made to design sealant systems from chemically synthesized materials which are easier to manufacture and control the desired characteristics and qualities and which have less potential for causing adverse biological reactions. For example, U.S. Pat. No. 4,826,945 to Cohn et al. discloses synthetically produced resorbable block copolymers of poly(xcex1-hydroxy-carboxylic acid)/poly(oxyalkylene) which are used to make absorbable sutures, wound and burn dressings and partially or totally biodegradable vascular grafts. These copolymers, however, are not crosslinked. The poly(alkylene) segments of such bio-absorbable copolymers are disclosed to be water-soluble so that the body can excrete the degraded block copolymer compositions. See also, Younes, H. and Cohn, D., J. Biomed. Mater. Res. 21, 1301-1316 (1987) and Cohn, D. and Younes, H., J Biomed. Mater. Res. 22, 993-1009 (1988). As set forth above, these compositions are uncrosslinked and, as a consequence, are relatively quickly bio-absorbed. Moreover, these uncrosslinked compositions generally require the presence of crystalline segments to retain their hydrogel-like consistency. As a result of such crystalline segments, these compositions have limited utility as sealants for vascular grafts.
Furthermore, U.S. Pat. No. 4,438,253 to Casey et al. discloses tri-block copolymers produced from the transesterification of poly(glycolic acid) and an hydroxyl-ended poly(alkylene glycol). Such compositions are disclosed for use as resorbable monofilament sutures. The flexibility of such compositions is controlled by the incorporation of an aromatic orthocarbonate, such as, tetra-p-tolyl orthocarbonate into the copolymer structure. The strength and flexibility which makes such a composition useful as a suture, however, does not necessarily make it appropriate for use as a sealant for a porous implantable prosthesis. Moreover, these tri-block copolymers are substantially uncross-linked. Thus, while compositions are somewhat hydrophilic, they do not form hydrogels.
Accordingly, attempts have been made to engineer bio-compatible hydrogel compositions whose integrity can be controlled through crosslinking. For example, U.S. Pat. Nos. 5,410,016 and 5,529,914 to Hubbell et al. disclose water-soluble systems which when crosslinked utilize block copolymers having a water-soluble central block segment sandwiched between two hydrolytically labile extensions. Such copolymers are further end-capped with photopolymerizable acrylate functionalities. When crosslinked, these systems become hydrogels. The water soluble central block of such copolymers can include poly(ethylene glycol); whereas, the hydrolytically labile extensions can be a poly(a-hydroxy acid), such as, polyglycolic acid or polylactic acid. See, Sawhney, A. S., Pathak, C. P., Hubbell, J. A., Macromolecules 1993, 26, 581-587.
Furthermore, U.S. Pat. No. 5,202,413 to Spinu discloses biodegradable multi-block copolymers having sequentially ordered blocks of polylactide and/or polyglycolide produced by ring-opening polymerization of lactide and/or glycolide onto either an oligomeric diol or a diamine residue followed by chain extension with a di-functional compound, such as, a diisocyanate, diacylchloride or dichlorosilane. The general structure of such a composition is R-(A-B-A-L)x-A-B-A-R, where A is a polyhydroxy acid, such as polylactide, polyglycolide or a copolymer thereof, B is an oligomeric diol or diamine residue, L is a diacyl residue derived from an aromatic diacyl halide or diisocyanate and R is H or an end-capping group, such as an acyl radical. A major difference between the compositions set forth in the Spinu ""413 patent and those described by the Cohn references supra is that Spinu uses lactide blocks whereas Cohn uses lactic acid blocks. Furthermore, like the Cohn copolymers, the copolymers described in the Spinu ""413 patent are not crosslinkable.
In general, all of the synthetic compositions set forth above describe copolymers having one or more segments which are water-soluble. Accordingly, many of the compositions described by these references are intended to be rapidly biodegraded by the body.
Thus, there is a need for water-insoluble, fully crosslinkable polymeric materials which are easily synthesized and provide controlled bioresorption in vivo. Moreover, there is a need for improved, cost-efficient synthetic sealant compositions for porous implantable prostheses which are characterized by their ability to self-emulsify and form stable low viscosity emulsions. There is a further need for sealant compositions which are quickly cured, exist as hydrogels in an aqueous environment and which remain flexible while dehydrated without the need for an external plasticizer. The present invention is directed to meeting these and other needs.
In one embodiment of the present invention, there is provided a covalently crosslinkable composition. This composition includes a water-insoluble copolymer which has a bioresorbable region, a hydrophilic region and a plurality of crosslinkable functional groups per polymer chain.
In another embodiment of the present invention, there is provided a medical device which has on at least one surface thereof a bioresorbable coating composition. This composition includes a hydrogel formed from the crosslinking of a polymer containing a bioresorbable region, a hydrophilic region, a plurality of crosslinkable functional groups and a crosslinking agent.
In a further embodiment of the present invention, there is provided a process for forming a hydrogel. This process includes providing an aqueous emulsion of a water-insoluble copolymer. This water-insoluble copolymer includes a bioresorbable region, a hydrophilic region, a plurality of crosslinkable functional groups per polymer chain and a crosslinking agent. Activation of the crosslinking agent crosslinks the copolymer composition and forms the hydrogel.
In yet a further embodiment of the present invention, there is provided a process for forming a medical device coated with a hydrogel. The hydrogel is formed from an aqueous emulsion which includes a water-insoluble copolymer having a bioresorbable region, a hydrophilic region, a plurality of crosslinkable functional groups per polymer chain and a crosslinking agent. This process includes applying the hydrogel to the medical device and then activating the crosslinking agent in a humid environment.
The present invention is directed to covalently crosslinkable compositions formed from water-insoluble copolymers. The copolymers of the present invention include a bioresorbable region, a hydrophilic region and a plurality of crosslinkable functional groups per polymer chain.
When uncrosslinked, the copolymer compositions form stable aqueous emulsions. Once crosslinked, however, such compositions form hydrogels in the presence of water. Hydrogels formed from the compositions of the present invention can serve as coatings for a medical device and/or as a therapeutic agent delivery vehicle.
The copolymers of the present invention are multi-block copolymers including, for example, di-block copolymers, tri-block copolymers, star copolymers, and the like. For purposes of illustration only, a typical tri-block copolymer of the present invention may have the following general formula:
xABAxxe2x80x83xe2x80x83(I)
wherein A is the bioresorbable region, B is the hydrophilic region and x is the crosslinkable functional group.
A more specific example of a copolymer useful in the present inventive compositions has the following chemical structure: 
wherein x is from about 10 to about 50 and y is from about 50 to about 300, so long as the composition remains water-insoluble as a whole.
One required feature of the present invention is that the crosslinkable copolymer composition be water-insoluble. For purposes of the present invention, xe2x80x9cwater-insolublexe2x80x9d is intended to mean that the copolymers of the present invention are substantially insoluble in water or water-containing environments. Thus, although certain regions or segments of the copolymer may be hydrophilic or even water-soluble, however, the copolymer molecule, as a whole, does not by any substantial measure dissolve in water or water-containing environments.
As set forth above, the water-insoluble copolymer includes a bioresorbable region. For purposes of the present invention, the term xe2x80x9cbioresorbablexe2x80x9d means that this region is capable of being metabolized or broken down and resorbed and/or eliminated through normal excretory routes by the body. Such metabolites or break-down products should be substantially non-toxic to the body.
The bioresorbable region is preferably hydrophobic. In another preferred embodiment, however, the bioresorbable region may be designed to be hydrophilic so long as the copolymer composition as a whole is not rendered water-soluble. Thus, the bioresorbable region is designed based on the requirement that the copolymer, as a whole, must remain water-insoluble.
Accordingly, the relative properties, i.e., the kinds of functional groups contained by, and the relative proportions of the bioresorbable region, and the hydrophilic region are selected to ensure that the present compositions remain water-insoluble.
The copolymers of the present invention form a stable aqueous emulsion. For purposes of the present invention, the terms xe2x80x9cemulsionxe2x80x9d, xe2x80x9cemulsifiablexe2x80x9d and xe2x80x9cself-emulsifyingxe2x80x9d refer to the ability of the copolymers of the present composition to form an emulsion, i.e., a colloidal suspension of one liquid in another, without the requirement of an emulsifying agent to stabilize the emulsion. Although emulsifying agents are not required by the present invention, their use is not excluded in appropriate circumstances is so desired by the skilled artisan. The relative proportions or ratios of the bioresorbable and hydrophilic regions, respectively are specifically selected to render the block copolymer composition water-insoluble. Furthermore these compositions are sufficiently hydrophilic to form a hydrogel in aqueous environments when crosslinked. Such hydrogels, as set forth in more detail below, can form a fluid-tight barrier when applied to a medical device. The specific ratio of the two regions of the block copolymer composition of the present invention will of course vary depending upon the intended application and will be affected by the desired physical properties of the porous implantable prosthesis, the site of implantation, as well as other factors. For example, the composition of the present invention remains substantially water-insoluble when the ratio of the water-insoluble region to the hydrophilic region is from about 10:1 to about 1;1, on a percent by weight basis.
The bioresorbable region of the present invention can be designed to be hydrolytically and/or enzymatically cleavable. For purposes of the present invention, xe2x80x9chydrolytically cleavablexe2x80x9d refers to the susceptibility of the copolymer, especially the bioresorbable region, to hydrolysis in water or a water-containing environment. Similarly, xe2x80x9cenzymatically cleavablexe2x80x9d as used herein refers to the susceptibility of the copolymer, especially the bioresorbable region, to cleavage by endogenous or exogenous enzymes.
Based on the characteristics set forth above, a number of different compositions can be utilized as the bioresorbable region. Thus, the bioresorbable region includes without limitation, for example, poly(esters), poly(hydroxy acids), poly(lactones), poly(amides), poly(ester-amides), poly(amino acids), poly(anhydrides), poly(ortho-esters), poly(carbonates), poly(phosphazines), poly(thioesters), polysaccharides and mixtures thereof. Furthermore, the bioresorbable region can also be, for example, a poly(hydroxy) acid including poly (xcex1-hydroxy) acids and poly (xcex2-hydroxy) acids. Such poly(hydroxy) acids include, for example, polylactic acid, polyglycolic acid, polycaproic acid, polybutyric acid, polyvaleric acid and copolymers and mixtures thereof
As set forth above, the present composition also includes a hydrophilic region. For purposes of the present invention, xe2x80x9chydrophilicxe2x80x9d is used in the classical sense of a material or substance having an affinity for water. Although the present composition contains an hydrophilic region, this region is designed and/or selected so that the composition as a whole, remains water-insoluble at all times.
When placed within the body, the hydrophilic region can be processed into excretable and/or metabolizable fragments. Thus, the hydrophilic region can include without limitation, for example polyethers, polyalkylene oxides, polyols, poly(vinyl pyrrolidine), poly(vinyl alcohol), poly(alkyl oxazolines), polysaccharides, carbohydrates, peptides, proteins and copolymers and mixtures thereof. Furthermore, the hydrophilic region can also be, for example, a poly(alkylene) oxide. Such poly(alkylene) oxides can include, for example, poly(ethylene) oxide, poly(propylene) oxide and mixtures and copolymers thereof.
As set forth above, the composition of the present invention also includes a plurality of crosslinkable functional groups. Any crosslinkable functional group can be incorporated into the present compositions so long as it permits or facilitates the formation of a hydrogel. Preferably, the crosslinkable functional groups of the present invention are olefinically unsaturated groups. Suitable olefinically unsaturated functional groups include without limitation, for example, acrylates, methacrylates, butenates, maleates, allyl ethers, allyl thioesters and N-allyl carbamates. Preferably, the crosslinking agent is a free radical initiator, such as for example, 2,2xe2x80x2-Azobis (N,Nxe2x80x2dimethyleneisobutyramidine) dihydrochloride.
The crosslinkable functional groups can be present at any point along the polymer chain of the present composition so long as their location does not interfere with the intended function thereof. Furthermore, the crosslinkable functional groups can be present in the polymer chain of the present invention in numbers greater than two, so long as the intended function of the present composition is not compromised.
Preferably, however, at least two olefinically unsaturated functional groups are present on the polymer chain of the present composition. As set forth above, the number of olefinically unsaturated functional groups present on the polymer chain can be increased beyond two, depending upon the particular application. Although the olefinically unsaturated functional groups can be positioned anywhere within the polymer chain of the present composition, it is preferred that at least one olefinically unsaturated functional group be positioned at a terminus of the polymer chain. More preferably, an olefinically unsaturated group is positioned at both terminal ends of the polymer chain. Furthermore, as there are at least two functional groups present in the present composition, the functional groups contained therein can be the same or different.
Crosslinking of compositions of the present invention is accomplished through the crosslinkable functional groups. These functional groups are activated to crosslink the copolymer composition by a variety of crosslinking initiators. These crosslinking initiators can include, for example, high energy radiation, thermal radiation and/or visible light. The composition of the present invention can also include free radical initiators. Such free radical initiators can include, for example, a peroxide or an azo compound.
In the present invention, the composition is crosslinked in an aqueous medium. Furthermore, when crosslinked, the copolymer composition is able to form a hydrogel. The hydrogels of the present invention are polymeric materials that swell in water without dissolving and that retain a significant amount of water in their structures. Such compositions have properties intermediate between liquid and solid states. Hydrogels also deform elastically and recover, yet will often flow at higher stresses. Thus, for purposes of this invention hydrogels are water-swollen, three-dimensional networks of hydrophilic polymers. These hydrogel compositions are not as transient as, and are more controllable than the prior art non-crosslinked sealant compositions described above. Thus, the present compositions have distinct advantages over the prior art and are able to function as superior sealants, for example, porous implantable prostheses, as well as, delivery devices for certain therapeutic agents.
In one embodiment of the invention, a therapeutic agent, such as for example a drug or bio-active agent, may be incorporated into the composition of the present invention for controlled release as the composition is bioresorbed. Thus, the present composition can be used to target therapeutic agents to specific sites in the body. Furthermore, the present composition can be engineered to bioresorb at a certain rate by controlling the ratio of the bioresorbable to the hydrophilic regions, as well as by controlling the degree of crosslinking thereof. Thus, the present compositions are able to delivery controlled quantities of a therapeutic agent to a specific site in the body as the block copolymer is bioresorbed.
Any drug or bio-active agent may be incorporated into the composition of the present invention provided that it does not interfere with the required characteristics and functions of the composition. Examples of suitable drugs or bio-active agents may include, for example, without limitation, thrombo-resistant agents, antibiotic agents, anti-tumor agents, antiviral agents, anti-angiogenic agents, angiogenic agents, anti-inflammatory agents, cell cycle regulating agents, their homologs, derivatives, fragments, pharmaceutical salts and combinations thereof.
Useful thrombo-resistant agents can include, for example, heparin, heparin sulfate, hirudin, hyaluronic acid, chondroitin sulfate, dermatan sulfate, keratin sulfate, lytic agents, including urokinase and streptokinase their homologs, analogs, fragments, derivatives and pharmaceutical salts thereof.
Useful antibiotics can include, for example, penicillins, cephalosporins, vancomycins, aminoglycosides, quinolones, polymyxins, erythromycins, tetracyclines, chloramphenicols, clindamycins, lincomycins, sulfonamides their homologs, analogs, fragments, derivatives, pharmaceutical salts and mixtures thereof.
Useful anti-tumor agents can include, for example, paclitaxel, docetaxel, alkylating agents including mechlorethamine, chlorambucil, cyclophosphamide, melphalan and ifosfamide; antimetabolites including methotrexate, 6-mercaptopurine, 5-fluorouracil and cytarabine; plant alkaloids including vinblastine, vincristine and etoposide; antibiotics including doxorubicin, daunomycin, bleomycin, and mitomycin; nitrosureas including carmustine and lomustine; inorganic ions including cisplatin; biological response modifiers including interferon; enzymes including asparaginase; and hormones including tamoxifen and flutamide their homologs, analogs, fragments, derivatives, pharmaceutical salts and mixtures thereof.
Useful anti-viral agents can include, for example, amantadines, rimantadines, ribavirins, idoxuridines, vidarabines, trifluridines, acyclovirs, ganciclovirs, zidovudines, foscamets, interferons their homologs, analogs, fragments, derivatives, pharmaceutical salts and mixtures thereof.
In another embodiment of the present invention, there is provided a medical device having on at least one surface thereof a bioresorbable coating composition. This coating composition includes a hydrogel which is formed from the crosslinking of a polymer containing a bioresorbable region, a hydrophilic region, a plurality of crosslinked functional groups and a crosslinking agent, as set forth previously.
The bioresorbable coating composition of the present invention can be applied as coatings to medical devices. In particular, the present bioresorbable coating compositions are intended to coat medical devices made from implantable materials. These bioresorbable coatings are capable of rendering fluid-tight porous medical devices such as conduits, vascular grafts, textile materials, polymeric films and the like. For purposes of the present invention, the term xe2x80x9cfluid-tightxe2x80x9d refers to the specific porosity of a material, such as a porous vascular or endovascular graft. Porosity of textile materials is often measured with a Wesolowski Porosity tester. With this apparatus, a graft is tied off at one end and the free end is attached to a valve on a porometer so that the graft hangs freely in a vertical position. Then, water is run through the graft for one minute and the water that escapes from the graft is collected and measured. The specific porosity of the graft is then calculated according to the following formula:   P  =      V    A  
where V is the volume of water collected in ml/min and A is the surface area of the graft exposed to water in cm2. A specific porosity of xe2x89xa61.0 ml/min/cm2 is considered an acceptable amount of leakage for an implantable vascular graft. Accordingly, for purposes of this invention, a substantially fluid-tight graft means a graft with a specific porosity, after impregnation with a sealant of the present invention, of xe2x89xa61.0 ml/min/cm2. Porosities meeting and exceeding the acceptable specific porosity criteria set forth above are achieved through the use of certain block copolymers described herein having polyether-polyester segments.
Implantable materials useful in the present invention can include, for example polymeric compositions, non-polymeric compositions and combinations thereof. The polymeric material can include, for example, olefin polymers including polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, fluorinated ethylene propylene copolymer, polyvinyl acetate, polystyrene, poly(ethylene terephthalate), polyurethane, polyurea, silicone rubbers, polyamides, polycarbonates, polyaldehydes, natural rubbers, polyester copolymers, styrene-butadiene copolymers and combinations thereof. Non-polymeric implantable materials can include, for example, ceramics, metals, inorganic glasses, pyrolytic carbon and combinations thereof. The compositions set forth hereinabove for the implantable substrate material of the present invention are intended to be exemplary only and should not be construed to limit in any way the types of materials to which the present bioresorbable coatings can be applied.
As set forth above, these implantable materials are used to manufacture medical devices, such as for example, endoprostheses. Grafts, stents and combination graft-stent devices are contemplated. Preferably these medical devices are vascular or endovascular grafts. Useful vascular or endovascular grafts include those which are knitted, braided or woven textiles, and may have velour or double velour surfaces. Alteratively, the medical device can be manufactured from an extruded polymer, such as polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), fluorinated ethylene propylene copolymer (FEP), polyurethane, silicone and the like. Composite structures are also contemplated.
In another preferred embodiment, the medical device may be a catheter, a guidewire, a trocar, an introducer sheath or the like. When coated onto such devices, the composition of the present invention imparts increased bio-compatibility to one or more surfaces thereof. Furthermore, when the present composition includes a drug or bio-active agent, specific therapeutic effects can be imparted to the surfaces of such devices. Moreover, the hydrophilic region of the present composition can impart increased lubriciousness to the surfaces of, e.g., a guidewire or other similar device.
Thus, any medical device to which the bioresorbable coating composition can adhere can be used in conjunction with the present invention. Accordingly, the examples of implantable materials and medical devices set forth hereinabove are for purposes of illustration only and are not intended to limit the scope of the materials and devices to which the present bioresorbable coatings can be applied or otherwise associated therewith.
In another embodiment of the present invention, there is provided a process for forming a hydrogel. This process includes: (I) providing an aqueous emulsion of a water-insoluble copolymer which contains a bioresorbable region, a hydrophilic region, a plurality of crosslinkable functional groups per polymer chain and a crosslinking agent; and (ii) activating the crosslinking agent, as set forth previously. In this process, the crosslinkable functional groups can be, but are not limited to, olefinically unsaturated groups. As set forth previously, the crosslinking agent can be a free radical initiator, an azo or a peroxide composition. Still further, the crosslinking agent can be, for example, thermally or photochemically activated.
In yet another embodiment of the present invention, there is provided a process for forming a medical device coated with a hydrogel. As set forth previously, this hydrogel is formed from an aqueous emulsion which includes a water-insoluble copolymer. This copolymer includes a bioresorbable region, a hydrophilic region, a plurality of crosslinkable functional groups per polymer chain and a crosslinking agent. Accordingly, this process includes applying the hydrogel to the medical device and then activating the crosslinking agent in a humid environment.
Although, the crosslinking agent can be activated in both humid and non-humid environments, it is preferred that the activation take place in humid environments. Preferably, the humid environment contains from about 20% to about 100% water. More preferably, the humid environment contains from about 60% to about 100% water.
The hydrogels formed by this process can be packaged and stored in a variety of ways. For example, the hydrogel can be maintained in a hydrated state for an extended period of time. Alternatively, the hydrogel can be dehydrated and stored in an essentially desiccated state until use.
As set forth previously, a therapeutic agent, such as for example, a drug or bio-active agent can be added to the emulsion for targeted, timed release of such agents in the body. Examples of types of therapeutic agents which can be incorporated into the emulsion have been set forth above.